import numpy as np
arr = np.array([10, 20, 30, 40, 50])
print(arr[0])    # 输出: 10
print(arr[1:4])  # 输出: [20 30 40] (切片，注意是左闭右开)
print(arr[::2])  # 输出: [10 30 50] (步长为2)